The present invention relates to power systems. More specifically, the present invention relates to thermal regulation of power systems.
A wide variety of electrical power converters are available to convert electrical power that is in one form, such as an AC supply, into another form, such as one or more DC voltages. The efficient performance of an electrical power converter depends on many factors, including the operating temperature of the converter. An excessive operating temperature can lead to a number of problems in the performance of a power converter. For example, operating at an excessive temperature can result in the power converter having a shorter lifetime or an increased likelihood of "thermal shutdown." In practice, the mean-time-between-failure ("MTBF") of a power converter is roughly proportional to the exponential of the temperature at which the converter is operated.
Power converters commonly are coupled in parallel to provide increased power to a single load. In such cases, it is desirable to equalize the power output and the temperature of the converters in the system so as not to overly burden one or more of the power converters in the system. However, it is difficult to control the temperature of each power converter within a system of parallel converters. For example, where a number of power converters are cooled by a fan, a temperature gradient may develop which causes the power converters relatively far from the fan to operate at a higher temperature. This temperature gradient can be further increased by possible differences in the power output of individual power converters in the system since an increase in the power output of a power converter tends to increase the temperature of the power converter.
To assist in regulating the temperature of the power converters in a system of parallel power converters, control circuits for balancing the power supplied by each power converter to the load may be used. In one type of control circuit, each power converter compares its own output current with the average output current of all the power converters in the system and changes its output current such that its output current approximates the average output current of the power converters in the system. Such a control circuit is commonly used in an N+1 power system, where N is an integer greater than one and represents the number of power converters needed to meet the power requirements of the load. An additional power converter is used in the N+1 system so that the power system can meet the power requirement of the load even when one of the power converters is not operating. One of the disadvantages of power sharing based only on the current output of the power converters is that the temperature of one or more power converters may still rise to a level that will reduce the MTBF of the power converter and/or bring it into thermal shutdown as a result of temperature imbalances that are caused by factors other than the imbalance in power supplied to the load.
Another type of control circuit, that is also used in N+1 power systems, compares the temperature of each power converter to the average temperature of the power converters in the system and adjusts the power output of the power converters so that the temperature of each power converter most closely approximates the average temperature of the power converters in the system. This temperature share scheme, at least in theory, increases the likelihood that the power converters in the system are all maintained at substantially the same temperature. When the temperature of a power converter is below the average temperature of the power converters in the system, then that power converter is caused to increase its power output. Similarly a power converter with an operating temperature that is higher than the average temperature of the power converters in the system is caused to decrease its power output.
A temperature share scheme, such as the one described in the preceding paragraph, that relies only on the temperature of the power converters to control the power output of a converter creates severe problems when a cold power converter is hot plugged into a power system with hot power converters. A hot plugged power converter will almost invariably be much colder than the other power converters in the system. Therefore, the temperature of the hot plugged power converter will be much lower than the average temperature of the power converters in the system. The large temperature differential between the cold and the hot converters would cause the output voltage of the cold converter to increase substantially in order to eliminate the large temperature differential. Thus, a power converter that is significantly colder than the other power converters in the system will make a significant input to the bus output voltage despite the fact that the output current of that particular converter is insignificant. Therefore, the hot plug in of a cold power converter causes a large change in the output bus voltage of the system.
Additionally, operating all the power converters in the system at the same temperature does not insure that each power converter will contribute an equal amount of power to the load. For example, a power converter that is better cooled may supply a greater than average amount of power to the load and still have about the same temperature as a power converter that is not as well cooled and provides less than the average amount of power to the load.
Some temperature share schemes, such as that disclosed in U.S. Pat. No. 5,493,154 issued to Smith, et al., (Smith, et al.,) control the output power of each power converter in a system of parallel power converters as a function of both the output current of each converter and the temperature of each converter such that the temperature of each converter approximates the average temperature of the power converters in the system. The use of both the current output in addition to the operating temperature of the power converter to control the power output of a power converter increases the likelihood that each of the power converters contributes an equal amount of power to the load and is operated at about the average temperature of the power converters in the system. However, in Smith, et al., the temperature and the output current of a power converter independently control the power output of the power converter, which as explained below creates certain problems particularly when hot plugging a cold power converter into a system of hot power converters.
The laplace transform of the temperature and current feedback in the Smith, et al., system is as follows: EQU T.sub.FEEDBACK +I.sub.FEEDBACK =(T/K1)/(S+P1)!+(I)*(K2)*(S+Z1)/(S+P1)!
where:
T.sub.FEEDBACK is the feedback component due to the temperature of the power converter; PA1 I.sub.FEEDBACK is the feedback component due to the current output of the power converter; PA1 T is the operating temperature of the power converter; PA1 I is the load current, i.e., the output current of the power converter; PA1 K1 and K2 are constants that are a function of the value of the components used in the power module, such as resistors in the control circuit, number of turns in the transformer, etc.; PA1 S is the laplace transform operator; PA1 P1 is a pole of the transfer function; PA1 Z1 is a zero of the transfer function.
In the above laplace transform, the first term which is a function of the temperature of the converter is added to the second term which is a function of the output current of the transformer. Thus, the temperature and the output current of the power converter independently feed back and control the power output of the power converter. As a result, the temperature of the power converter can, independent of the output current of the power converter, control the power output of the power converter. This independence can cause severe problems when a cold power converter is hot plugged into a power system with hot power converters. As discussed above, a hot plugged power converter will almost invariably be much colder than the other power converters in the system. Therefore, the temperature of the hot plugged power converter will be much lower than the average temperature of the power converters in the system. The large temperature differential between the cold and the hot converters causes the output voltage of the cold converter to increase substantially in order to eliminate the large temperature differential. Thus, a power converter that is significantly colder than the other power converters in the system will make a significant input to the bus output voltage despite the fact that the output current of that particular converter is insignificant. Since the temperature and the output current independently control the power output, a given temperature differential will have substantially the same effect on the power output of the converter regardless of the output current of the power converter. Therefore, the hot plug in of a cold power converter may cause a large change in the output bus voltage of the system. Thus, the problems associated with the hot plug in of a cold power converter exists in the Smith, et al., system as it does in temperature share scheme systems that use only the temperature of the power converter to control the power output of the converter.
Additionally, all of the above control circuits rely on a share line to share common current or temperature information between the power converters. The share line allows averaging the operating temperatures or output currents of the power converters in the system and is often considered to be the most accurate means for power sharing between converters. However, use of the share line has disadvantages, one of which is the single point of failure. The single point of failure refers to the failure of the entire system to operate properly as a result of the failure of at least one of the power converters in the system. An example of such a failure includes the thermal shutdown of a power converter in the system. A power converter often will thermally shut down when it operates at a high temperature for a long period of time. A power converter is more likely to thermally shut down if it is hot plugged into a system. The newly plugged in power converter rapidly increases its power output in order to match the power output of the other power converters in the system. This rapid increase in power output may also cause a rapid increase in the temperature of the power converter. If the power output of the power converter is not controlled in time, then the power converter may continue to heat up to a point where it eventually thermally shuts down. When a power converter in a share line system thermally shuts down, then the share line to which the power converter is connected may be shorted as a result of possible shorts in the thermally shut down power converter. Unfortunately, there are no known ways of insuring operability of the power system when the external share line is shorted. The shorting of the share line removes the only control means for controlling the power output of the power converters in the system. In the absence of a means for controlling the power output of the power converters in the system, the likelihood of other power converters in the system and eventually the entire power system thermally shutting down increases.
One way to overcome the single point of failure problem is to use what is referred to as a 2N redundant power system, where N is an integer greater than one and is also the number of power converters needed to meet the power requirements of the load. In such a system, a first group of N power converters, i.e., a first bay (or array) of power converters, are coupled to each other by a first share line while the second group of N power converters, i.e. a second bay (or array) of power converters, are coupled to each other by a second share line. Both the first and second groups of power converters are coupled to the same load. The first and the second share lines are independent of each other such that the shorting of one share line does not necessarily cause a shorting of the other share line. When the share line in one bay is shorted, then the N power converters in the other bay provide the necessary power to the load. Thus, the 2N redundant power system effectively deals with the single point of failure problem present in power system using a share line.
However, the 2N redundant power system does not efficiently utilize all of the power converters in the system. Invariably one bay will have a higher output voltage than the other bay and would, therefore, dominate the output by providing a higher output voltage while the other bay would be off. Therefore, there is no power sharing at all between the power converters in the separate bays. Thus, the single point of failure is avoided at the cost of having twice as many power converters as are needed to supply the necessary power to the load since half of the power converters are idle at any given time.
Another prior art power sharing scheme, namely a down slope power sharing system, avoids the problem of a single point of failure by simply not using a share line. The down slope curve of a power module is typically defined as a plot of the output voltage of a power module as a function of the output current of the same module. In a down slope power sharing system, power sharing is accomplished by designing the power modules in the system to have a substantially equal impedance, and then coupling the output of each of the power modules to the same output line such that the output voltage of each of the power modules is substantially identical. Given that the impedance and the voltage of all the power modules are substantially identical, the output current of each of the power modules, according to Ohm's law, will be substantially identical as well. In this system, therefore, the down slope curve for each of the power modules in the system is substantially identical.
However, even in a down slope power sharing system, the power modules in the system may be operating at different temperatures since they may not all be exposed to identical cooling influences. For example, a power module that is farther removed from a cooling fan is less likely to be cooled down to the same temperature as a power module situated closer to the cooling fan. Therefore, it is possible, and in many cases likely, in a down slope power sharing system for the power modules to be outputting an equal amount of power even though they are operating at different temperatures. The power modules operating at a higher temperature have a lower MTBF, which as noted above is roughly proportional to the exponential of the temperature at which the converter is operated. Therefore, the power modules operating at a higher temperature are more likely to thermally shut down before the power modules operating at a lower temperature. The thermal shutdown of one power module in the system increases the share of the power output that each of the other power modules in the system must contribute to the load so as to meet the power requirement of the load. This increases the likelihood of other power modules and eventually the entire power system thermally shutting down.
Therefore, it is desirable to thermally regulate the power sharing of power modules in a down slope power sharing system such that power modules operating at a higher temperature compared to other power modules in the system are caused to output a lower current for a given output voltage.
Thermal regulation of the power output of power modules is particularly useful in a power system with a 48 volt or lower voltage bus. Power modules supplying power to a 48 volt or lower voltage bus tend to have a higher power conversion rate and lower thermal capacity per individual power module as compared to larger and generally greater thermal capacity discrete designs. Therefore, the power modules supplying power to a 48 volt or lower voltage bus, such as 24 volts or 12 volts, are more likely to heat up and thermally shut down.